Recombinant human Epo (rEpo) has become available and procedures have been developed whereby erythroid progenitor cells of several provenances can be harvested in near purity. Our preliminary studies of Epo-reception by ca. 80%-pure murine erythroid colony-forming units (CFUe) and by the S8 murine erythroleukemia (MEL) cell line, together with collateral studies by others of purified spleen cells derived from mice infected with the anemia-strain of Friend virus (FVA), indicate that there are at least two kinds of erythropoietin receptors (EpoR): one with a dissociation constant (Kd) of approximately 40 pM, the other with Kd of approximately 400 pM. Conditioned by our repeated observation that erythroid maturation can be atypical in virus- infected cells, we hypothesize that (i) high-affinity (approximately 40 pM) EpoR is either a viral import or one regulated by virus transformation and (ii) low-affinity (approximately 400 pM) EpoR is the one naturally expressed in mouse CFUe. Our intent is to verify/falsify these hypotheses and, concurrently, answer questions arising from preliminary studies of Epo-response by CFUe and from studies of EpoR biology in CFUe and S8-MEL-cells. We hope thereby to clarify the temporal relationship between Epo-addition and globin gene response; resolve the mystery of why CFUe-EpoR sites seem abundant far beyond functional needs; establish some ways in which EpoR can be regulated; refine our understanding of physical features and fate of EpoR, including confirmation/rejection of the possibility that Epo and/or EpoR is bound to DNA or chromatin in S8-MEL cells; in a large-scale undertaking, apply affinity methods to the isolation of EpoR and immunological methods to the isolation of recombinant cDNA clones encoding it; and, in a brisk but short- lived undertaking, isolate cDNA for "early-response" genes whose expression depends upon Epo and, thereafter, use arising cDNA clones to examine heretofore unknown aspects of Epo-responses by the several EpoR.